FR2717472A1 - High temperature coating, in two layers, on a ceramic substrate, its production and its applications. - Google Patents

Abstract

The high temperature coating according to the invention comprises a barrier (primer) layer and an emittance enamelling (finishing) layer, of compositions by weight: barrier layer: - silicon tetraboride (more than 96% by weight of SiB4 ): 0.1 to 10%; - quartz glass: 90 to 99.9%; emittance enamel layer: - silicon tetraboride (more than 96% by weight of SiB4): 1.5 to 5.0%; - glass with high silica content: 95 to 98.5%. Application to the protection of ceramic materials, in particular porous, against erosion and chemical and / or mechanical failures.

Description

The invention relates to materials, more specifically coatings, for

  the protection of ceramic substrates, in particular porous substrates, against erosion,

  chemical failures and mechanical failures.

  Fibrous ceramic materials based on refractory oxides operating at high temperatures are widely used. The conditions of use of these materials impose requirements on the coatings used for their protection: high temperature stability, resistance to erosion, thermochemical stability and

high phase stability.

  A variety of erosion-resistant coatings are now known which operate at temperatures up to 1260 C. A two-layer coating is known comprising a barrier layer and an enameling layer (see US Patent 3,953). 646). The barrier layer is comprised of a fused silica deposition comprising from about 80 to about 90 weight percent solids. The coating is applied to the substrate by spraying. The barrier layer is fired at a temperature of about 930 to about 1370 C. The enameling layer of high silica glass, borosilicate glass and emittance agent is applied to the barrier layer. . The emittance agent is chosen from the group consisting of silicon carbide, chromium, cobalt and nickel oxides, nickel-chromium spinels, silicon nitride and calcined iron-chromium mixed oxides. and / or nickel. High silica glass (Corning Glass No. 7913) contains not less than 94% by weight of SiO2. Borosilicate glass (Corning Glass No. 7740) has the following composition (by weight): 70 to 87% SiO 2, 10 to 20% B 2 O 3,

  2-5% Na2O, 1-5% Al2O3.

  The high silica content vitreous component and the borosilicate glass component are used in a weight ratio of about 3: 1 to about 19: 1, and glass components (high silica glass and borosilicate glass) ) and the emittance agent are used in a weight ratio of from 50: 1 to about 4: 1. An aqueous slip containing from about 10 to about 90% by weight of enameling coating is fired at a temperature

ranging from 930 to about 1370 C.

  It is known in the art (see US Pat. No. 3,955,034) a three-component coating for the isolation of silica, comprising a silica barrier layer, an emittance layer comprising a vitreous component having a high silica content and a emittance agent selected from the group consisting of silicon carbide, nickel oxide, chromium oxide, cobalt oxide, nickel-chromium spinel, silicon nitride, calcined mixed oxides of silicon, iron, chromium and cobalt, with a weight ratio of high silica glass to emittance agent of about 50: 1 to about 4: 1, and a coating layer of enameling consisting of high silica content and borosilicate type glass, in a weight ratio of borosilicate glass to high silica to borosilicate glass from about 3: 1 to about 19: 1. The coating is

  cooked at a temperature ranging from 930 to about 1370 C.

  These coatings provide neither thermal shock resistance nor thermal stability of the emittance

  sufficient, and they undergo a withdrawal.

  To overcome the problems mentioned above, it has been proposed a monolayer coating (see US Pat. No. 4,093,771) which is prepared by reacting a compound selected from the group of substances consisting of silicon tetraboride, silicon hexaboride , other boron silicides, boron and mixtures of these substances, with a reactive glass frit composed of porous borosilicate glass with high silica content, and boron oxide. A thin layer of borosilicate glass is formed on finely divided particles of high silica glass, which improves sintering of the coating without significant increase in

  coefficient of thermal expansion.

  The reactive glass sinter is suitably prepared by mixing about 2 to 10 parts by weight of boron oxide with 100 parts by weight of high silica porous borosilicate type glass, such as Vycon 7930 glass. High Silica Borosilicate Vycon 7930 has a porosity of approximately 28%. The boron oxide is dissolved in 400 parts by weight of deionized water. The mixture is stirred at about 95 ° C and then dried for up to 24 hours at 75-95 ° C. The resulting glass sinter is dispersed, sieved and baked at 1150 ° C for 1 hour. The resulting sintered composite is crushed

to give a powder and sieved.

  A typical composition would consist of 97.5% by weight of reactive glass sinter containing 5.5% by weight of boron oxide, combined with 2.5% by weight of silicon tetraboride composed of 63% by weight of silicon, 36% by weight boron and less than 0.2% by weight magnesium. The coating slip is prepared by mixing finely divided particles of reactive glass sinter and silicon tetraboride, with a carrier such as ethanol and a binder such as methylcellulose, with a proportion by weight of solid components of 35 to 50%. The mixture of coating components is ground in an alumina ball mill with alumina beads for 3 to 12 hours. The coating is applied by spraying. The coated samples are dried for 2 to 5 hours at temperatures in the range of 20 to about 70 ° C. After drying, the coated samples are enamelled in an oven for 1 1/2 hours at 1215 C. The coating has an emittance of approximately 0.90 to 0.93 from room temperature to above 1260 C. The coefficient of expansion

thermal is 1.10-6K-1.

  Also known is an improved low density coating for the protection of porous aluminosilicate materials having a service temperature of up to 1300 C. The coating composition comprises 77.5% by weight of reactive glass sinter 2.5% by weight of silicon tetraboride and 20% by weight of molybdenum disilicide. The coating is formed on the substrate at 1230 C for 1 1/2 hours (see: Advanced Porous Coating for Low Density Ceramic Insulation Materials,

  J. Amer. Ceram. Soc., Vol. 72, No. 6, pp. 1003-1010, 1989).

  The use of borosilicate glass with a high silica content and having an active surface in the coating composition can cause a decrease in thermochemical stability and stability.

phase.

  The phase transformations in the coating are related to the formation of cristobalite which causes

cracking of the coating.

  The present invention provides a high temperature coating on a ceramic substrate which does not have the disadvantages of the prior art coatings. Indeed, the coating according to the invention has a high thermochemical stability, high impact resistance

  thermal, very stable phases and low shrinkage.

  More specifically, the invention provides a high temperature coating on a ceramic substrate, in particular a porous substrate, characterized in that this coating comprises a barrier (primary) layer containing quartz glass and silicon tetraboride containing more than 96% by weight of SiB4. and an emittance enamelling (finishing) layer containing high silica content glass and silicon tetraboride greater than 96% by weight SiB4, which layers have the following compositions by weight: barrier layer: - tetraborate silicon (greater than 96% by weight of SiB4): 0.1 to 10%; quartz glass: 90 to 99.9%; emittance enameling layer: silicon tetraboride (greater than 96% by weight of SiB4): 1.5 to 5.0%;

  glass with a high silica content: 95 to 98.5%.

  The ceramic substrate is made of a ceramic material generally comprising one or more compounds selected from the group consisting of Al 2 O 3, ZrO 2, SiO 2, SiC and Si 3 N 4. Its density is generally greater than

kg / m3 (0.1 g / cm3).

  In the coating, the content of silicon tetraboric particles having a size of less than 5 μm is preferably 70 to 80% by weight of said silicon tetraboride, the SiO 2 content in the quartz glass is preferably 99%, 96% by weight and the high silica content glass advantageously comprises, by weight: 94 to 96% SiO 2; 3.5 to 6% B203; 0.1 to 0.5% of Al 2 O 3; and 0.1 to

0.5% Na2O.

  The thermochemical properties of glass with high silica content due to its specific composition, in particular silicon, aluminum and sodium oxides, its high purity and its specific dispersibility ensure the desired chemical interaction between the amorphous components (glass) and polycrystalline ceramic (silicon tetraboride) coating. As a result, during firing of the coated ceramic substrate, curing occurs in its emittance enamel reactive layer. This hardening makes it possible to substantially reduce the shrinkage of the ceramic substrate, to increase the resistance to thermal shock, and to increase the thermostability of the coating to stabilize

thermochemical properties.

  The presence of more than 98.5% by weight of high silica glass and less than 1.5% by weight of silicon tetraboride in the emittance enameling layer increases the softening temperature of the coating, which decreases its phase stability and emittance properties. If this layer comprises less than 95% by weight of glass with a high silica content and more than 5% by weight of silicon tetraborate, it has insufficient temperature resistance and thermochemical stability. A content of more than 80% by weight of silicon tetraboric particles having a size of less than 5 μm results in its ability to penetrate through the porous ceramic substrate substantially increases, leading to shrinkage of the latter. A particle content of this type of less than 70% by weight leads to a non-uniform local distribution of the silicon tetraboride through the vitreous matrix, which increases the stress of the coating and reduces its thickness.

thermal shock resistance.

  The moisture and dispersion of the silicon tetraboride are controlled. If the quartz glass content in the barrier layer is greater than 99.9% by weight, the interface between the barrier layer and the substrate has a density that is too low, which decreases the adhesion between the coating and the substrate and increases the penetrability of the

  Emittance enameling layer, which causes shrinkage.

  If the quartz glass content is less than 90% by weight, the density of the barrier layer increases substantially, which leads to its non-uniform impregnation by the composition of the slip intended to form the emittance enameling layer and the appearance

cracks in the latter.

  The use of high silica content borosilicate glass having an active surface in the coating composition could produce a decrease in its thermochemical stability and stability

phase.

  The phase transformations in the coating are related to the formation of cristobalite a, which causes

cracking of the coating.

  The penetration of the coating into the porous substrate provides high adhesion. A densified layer having a thickness of 70 to 140 am and a density of up to 500 kg / m 3 (0.5 g / cm 3) is generally formed on the substrate. The coating is advantageously applied by means of compressed air at a pressure of 0.8 × 10 5 to 1.1 × 10 5 Pa (0.8 to 1.1 atm). The weight ratio of dispersion phase (coating powder): dispersion medium (preferably distilled water) is from 1: 1 to 1: 5. A high content of dispersion medium, especially water leads to a chemical composition of the non-uniform coating. A low water content leads to a decrease in adhesion between the coating and the substrate. The coating is applied to the surface of the ceramic substrate that has been pretreated, for example by dusting the

  felt forming the substrate, for better adhesion.

  The firing of the (primary) barrier layer at temperatures of 1100 to 1150 C for 10 to 20 minutes and the firing of the emittance enameling layer (topcoat) at temperatures of 1250 to 1300 C for 10 to

  20 minutes reduce the shrinkage of the material.

  Removal of the coated material occurs at cooking temperatures above 1300 C and for cooking times longer than 20 minutes, while the emittance properties decrease with temperatures and

  cooking times below the above values.

  According to another of its aspects, the invention therefore relates to a method for providing a ceramic substrate, in particular porous, of the coating described above, characterized in that it essentially comprises the steps of: - preparing a first slip a powder consisting of 0.1 to 10% by weight of silicon tetraboride (greater than 96% by weight of SiB4) and 90 to 99.9% by weight of quartz glass, in a compatible dispersion medium preferably distilled water, with a weight ratio of powder to liquid of 1: 1 to 1: 5; - Apply, by spraying under pressure, this first slip on the ceramic substrate to be coated having undergone preparation treatment; drying the layer thus obtained and baking it at a temperature of 1100 to 1150 ° C for 10 to 20 minutes, to obtain a barrier layer; preparing a second slip of a powder consisting of 1.5 to 5.0% by weight of silicon tetraboride (greater than 96% by weight of SiB4) and 95 to 98.5% by weight of high-grade glass; silica content, in a compatible dispersion medium, preferably distilled water, with a weight ratio of powder to liquid of 1: 1 to 1: 5; - Apply, by spraying under pressure, this second slip on the barrier layer formed previously; drying the layer thus obtained and baking it at a temperature of 1250 to 1300 C for 10 to 20 minutes, for

  obtain an emittance enameling layer.

  The following examples are intended to illustrate

and better explain the invention.

  Exemole 1: A coating is prepared according to the techniques of

slip-baking coating.

  The barrier layer (primary layer) is first prepared. Quartz glass is milled in an alumina bead mill to give a powder having a surface area of 0.6 to 1 m 2 / g, and sieved. 95 parts by weight of quartz glass and 5 parts by weight of silicon tetraboride comprising 70% by weight of particles having a size less than 5 μm are mixed in a polyethylene container for 25 hours. A weighed amount of powder is diluted with distilled water in a weight ratio of 1: 1 and sprayed at 105 Pa (1 atm) air pressure onto the previously treated surface by dedusting a material. ceramic based on fibers. We

  applies a densified layer having a thickness of 100 μm.

  The sample is air-dried for 30 minutes and in an oven at 80 ° C for 30 minutes. The barrier layer (primary layer) is then fired at 1120 C for 15 minutes. The cooling emittance (finish coat) enameling layer is then applied to the

barrier layer.

  The emittance enameling layer is prepared from 95 parts by weight of high silica glass and 5 parts by weight of silicon tetraboride comprising 70% by weight of particles having a size of less than 5 gm. The glass is ground in an alumina bead mill to give a powder having a specific surface area of 0.6 to 1 m 2 / g, and sieved. The glass powders and silicon tetraborure are mixed in one

  polyethylene container for 48 hours.

  A weighed amount of powder is diluted with distilled water, in a weight ratio of 1: 3, and applied to the barrier layer (primer) by

  spray at an air pressure of 105 Pa (1.0 atm).

  The sample is air-dried at 20 ° C. for 30 minutes and oven-dried at 80 ° C. for 30 minutes. The enameling layer

  Emittance is baked at 1250 ° C. for 15 minutes.

Example 2

  The barrier layer is prepared according to the process of Example 1. It comprises 98% by weight of quartz glass and 2% by weight of silicon tetraborate containing% by weight of particles having a size less than 1 μm. The powder: water ratio is 1: 2 and the air pressure is 0.8.105 Pa (0.8 atm). The thickness of the densified layer obtained is 70 μm. The barrier layer is cooked

at 1150 C, for 10 minutes.

  The emittance enameling layer is prepared

according to the process of Example 1.

Example -3:

  The emittance enameling layer comprises 98% by weight of high silica content glass and 2% by weight of silicon tetraboride containing 80% by weight of

  particles having a size less than 5 gm.

  The emittance enameling layer (topcoat) is applied to a barrier layer (primer layer) prepared according to the process of Example 2 and fired at 1280 ° C for 10 minutes. The coating is subjected to tests of thermochemical stability, phase stability and erosion resistance in an air plasma. After 30 cycles at 1250 ° C., the cristobalite content a is not greater than 0.5% by weight.

weight.

  The thermochemical stability is evaluated by electron microscopy through the thickness of the porous layer (defective layer formed during the test).

  thermochemical stability) whose value is 30 gm.

  The integral emittance is 0.86, the coefficient of thermal expansion is 1.1 × 10 -6K -1. The coating does not

undergoes no withdrawal.

Claims (10)

  1. high temperature coating on a ceramic substrate, in particular porous, characterized in that this coating comprises a barrier (primary) layer containing quartz glass and silicon tetraborure with more than 96% by weight of SiB4, and a layer (of emittance enameling finish containing high silica content glass and silicon tetraboride greater than 96% by weight SiB 4, which layers have the following weight compositions: barrier layer: - silicon tetraborate (more than 96% by weight of SiB4): 0.1 to 10%; quartz glass: 90 to 99.9%; emittance enameling layer: - silicon tetraboride (greater than 96% by weight of SiB4): 1.5 to 5.0%;   glass with a high silica content: 95 to 98.5%.   2. The coating of claim 1, characterized in that it is in the form of a densified layer having a thickness of 70 to 140 m and in that   that its density can reach 0.5 g / cm3.   3. A method for providing a ceramic substrate, in particular porous, a coating according to claim 1 or 2, characterized in that it essentially comprises the steps of: - preparing a first slip of a powder consisting of 0.1 10% by weight of silicon tetraboride (greater than 96% by weight of SiB4) and 90 to 99.9% by weight of quartz glass, in a compatible dispersion medium, preferably distilled water, with a weight ratio of powder to liquid of 1: 1 to 1: 5; - Apply, by spraying under pressure, this first slip on the ceramic substrate to be coated having undergone preparation treatment; drying the layer thus obtained and baking at a temperature of 1100 to 1150 C for 10 to 20 minutes, to obtain a barrier layer; preparing a second slurry of a powder consisting of 1.5 to 5.0% by weight of silicon tetraboride (greater than 96% by weight of SiB4) and 95 to 98.5% by weight of high grade glass silica, in a compatible dispersion medium, preferably distilled water, with a weight ratio of powder to liquid of 1: 1 to 1: 5;   - Apply, by spraying under pressure, this second slip on the barrier layer formed previously; drying the layer thus obtained and baking it at a temperature of 1250 to 1300 C for 10 to 20 minutes, for   obtain an emittance enameling layer.   4. Method according to claim 3, characterized in that the ceramic substrate comprises one or more compounds selected from the group consisting of Al 2 O 3, ZrO 2, SiO 2, SiC and Si 3 N 4, and has a density greater than 0.1 g / cm3.   5. Method according to claim 3 or 4, characterized in that the content of particles of silicon tetraborure whose size is less than 5 gm, is 70   at 80% by weight of said silicon tetraborate.   6. Process according to any one of   Claims 3 to 5, characterized in that the content of   SiO 2 of the quartz glass is 99.96% by weight.   7. Process according to any one of   Claims 3 to 6, characterized in that the high-grade glass   silica content has the following composition by weight: SiO 2: 94 to 96% B 2 O 3: 3.5 to 6%   A1203: 0.1 to 0.5% Na2O: 0.1 to 0.5%.   8. Process according to any one of   Claims 3 to 7, characterized in that the layers of   coating are applied under air pressure of 0.8.105 to 1.105 Pa.   9. Application of the process according to any one   Claims 3 to 8 to the coating of materials   ceramics, especially porous, to protect them against erosion, chemical failures and mechanical failures. A ceramic material having the coating of claim 1 or 2, or as obtained by use of the   process according to any one of claims 3 to 8.